Cyclin dependent kinases (CDKs) are a family of serine/threonine kinases that are essential to cell cycle progression. Consequently, the activities of these kinases are tightly regulated. In mammals, at least seven different CDKs have been described to date and have been characterized as CDKs 1-7. They are well conserved, sharing 40 to 75% identity. In addition, extensive similarity has been shown with other serine/threonine protein kinases within their catalytic domains. See, Pines, J., Semin. Cell Biol. 1994, 5:399-408; Morgan, D. O., Nature, 1995, 374:131-134; and Nigg, N. A., Bioessays, 1995, 17:471-480. Various mechanisms to regulate CDK activity are used to ensure that the cell's normal cycle is tightly controlled and yet remains exquisitely sensitive to changes in the environment. Lees, E., Curr. Opin. Cell Biol. 1995, 7:773-780.
For example, entry of cells into mitosis is initiated by the M phase-promoting factor (MPF), a complex of the Cdc2 protein kinase and cyclin B. Proper regulation of MPF ensures that mitosis occurs only after earlier phases of the cell cycle are complete. Phosphorylation of Cdc2 at Tyr.sup.15 and Thr.sup.14 suppresses the activity of MPF during interphase. At G.sub.2 -M transition the Cdc2 is dephosphorylated at Tyr.sup.15 and Thr.sup.14 allowing MPF to phosphorylate its mitotic substrates.
A distinct family of Cdc-regulatory kinases, referred to as Wee-1, has been identified and characterized. Wee-1 was first identified in the fission yeast Schizosaccharomyces pombe as an important negative regulator of mitosis. Russell, P. and Nurse, P., Cell, 1987, 49:559. Homologs of Wee-1 have since been identified in at least six other organisms. In human and Xenopus, WEE-1 is a soluble enzyme that phosphorylates Cdc2 on Tyr.sup.15, but not on Thr.sup.14. Mueller et al., Mol. Biol. Cell, 1995, 6:119; McGowan, C. H. and Russell, P., EMBO J., 1993, 12:75: Parker, L. L. and Piwnica-Worms, H., Science, 1992, 257:1955; and McGowan, C. H. and Russell, P. EMBO J., 1995, 14:2166: Watnabe et al., ibid., p. 1878.
A Thr.sup.14 -specific kinase activity has been detected in the membrane fraction of Xenopus egg extracts. Atherton-Fessler et al. Mol. Cell Biol. 1994 5:989: Kornbluth et al., ibid., p.273. It has also been demonstrated using extracts of Xenopus eggs, that this Thr.sup.14 -specific kinase is tightly membrane associated. Kornbluth et al. Mol. Biol. Cell 1994 5:273-282. Further, the Thr.sup.14 -specific Cdc kinase, referred to as Xenopus Myt-1 membrane-associated inhibitory kinase, was recently shown to be an important regulator of Cdc2/cyclin B kinase activity. Mueller et al., Science, 1995, 270:86-90. Conceptual translation of the Xenopus gene encoding Myt-1 revealed that it is most similar to the Wee-1 family of kinases. Thus, Myt-1 is a subclass of the Wee-1 family.
Regulation of Myt-1 kinase offers a means of controlling a critical event in the cell cycle. Inhibition of Myt-1 kinase activity is believed to result in de-regulation of the timing for entry of cells into mitosis. This generally results in catastrophic mitosis and cell death due to the cells entry into mitosis before all essential proteins and/or DNA is produced. Thus, is it believed that inhibition of Myt-1 activity has utility in treating cancers, such as leukemias, solid tumors and metastases; chronic inflammatory proliferative diseases, such as psoriasis and rheumatoid arthritis; proliferative cardiovascular diseases, such as restenosis; proliferative ocular disorders, such as diabetic retinopathy and macular degeneration; and benign hyperproliferative diseases, such as benign prostatic hypertrophy and hemangiomas.
Clearly there is a need for identification and characterization of human homologs of Myt-1 kinase.